The present invention relates to connecting devices for high density multichip hybrid integrated circuit boards and, more particularly, to a method of making fineline flexible circuits for interconnecting miniaturized fine pitch connectors.
In todays miniaturized high-density multichip hybrid integrated circuit boards plug and socket connecting devices are no longer used. Instead, fineline flexible circuits having raised surface features such as bumps or dots are clamped in electrical contact with fine pitch etched circuit traces on the circuit boards. Interconnection systems of this type are described in U.S. Pat. No. 4,125,310 to Patrick A. Reardon, II; U.S. Pat. No. 4,116,517 to Selvin, et al.; and U.S. Pat. No. 4,453,795 to Moulin. The connectors of these patents have a plurality of metallic raised features that protrude or project from the plane of the circuit conductors. These raised features may be pressed against either similar raised features or mating conductive connecting pads or etched circuit traces on a circuit board. The two circuits may be physically clamped together to press the features against one another thereby making firm and intimate electrical contact between the two circuits. Such interconnects typically employ 0.003 inch diameter features on flexible circuits that connect to 0.005 inch wide etched circuit traces on the circuit boards. The interconnect density may be as high as 2800 interconnects per square inch.
Multilayer printed circuit boards often have an uneven or irregular surface in the region where the etched circuit traces are located. To make reliable connection, more stringent flatness requirements are required for the circuit board, or the complexity and weight of the clamping structure is increased to force a better connection between the wires in the flexible circuit and the etched traces on the printed circuit board. However, higher pressure exerted by the clamping structure tends to damage the substrate or to distort the flexible circuit so that the wires "swim" and registration of the fine pitch connections is lost.
Accordingly, it is an objective of the present invention to provide compliance to a fineline interconnection so that the flatness requirements of the opposing mating structure may be relaxed. Another objective of the present invention is the provision of a low cost, high density compliant interconnection system capable of compensating for device surface irregularities. A further objective of the present invention is to add compliance to any flexible circuit fabricated using conventional methods. A still further objective of the invention is to provide spring compliance to a fineline flexible circuit interconnection that permits reducing complexity and weight of a clamping structure used in conjunction therewith. Yet another objective of the present invention is the provision of an interconnection system that is more reliable, more compact and less costly. Still another objective of the invention is to provide an interconnection system that distributes the force required to make contact, thus reducing the possibility of damage to the substrate.